Aramid fibers are highly resistant to heat decomposition, have inherent flame resistance, and are frequently used in working wear for special environments where flame resistance is required. Fabrics made of these fibers are extremely strong and durable, and have been widely adopted for military applications where personnel have the potential to be exposed to fire and flame, such as aircraft pilots, tank crews and the like. There is a need for dyed fabrics that have flame-resistant properties even greater than the undyed fabrics or dyed fabrics. Meta-linked aromatic polyamide fibers (aramid fibers) are made from high-molecular-weight polymers that are highly crystalline and have either a high or no glass transition temperature.
These inherent desirable properties of aramid fibers also create difficulties for fiber processing in other areas, specifically, aramids are difficult to dye. Fiber suppliers currently recommend a complicated exhaust dyeing procedure with a high carrier (acetophenone) content; the process is conducted at high temperatures over long periods of time and often results in a product having an unpleasant odor. Such dyeing conditions require substantial amounts of energy both to maintain dyeing temperature and for the treatment of waste dye baths.
Polar organic solvents have also been used to swell the fiber or create voids in the fiber structure to enhance dyeability. These procedures involve solvent exhaust treatments at elevated temperatures with subsequent dyeing. Another source of dyed aramid fiber is solution-dyed aramid yarn, available from the producer, prepared by solution dyeing in which a quantity of dye or pigment is mixed with the molten or dissolved polymer prior to extrusion of the polymer or solution into fine fibers; the dye or pigment becomes part of the fiber structure. Solution-dyed fibers are more costly than the undyed fibers due, in part, to the additional costs of manufacture, and must be used in the color provided by the supplier, leaving the user with only a limited choice of colors. Solution-dyed fibers offer relatively good lightfastness, whereas some undyed aramid fibers, particularly Nomex.RTM. (DuPont), yellow following exposure to UV light. Because of this potential for yellowing, although deep, rich colorations, particularly dark blue and navy blue, are achievable, they still lack acceptable lightfastness.
More recently, a process has been described in U.S. Pat. No. 4,525,168 in which acid or anionic dyes are introduced into aramid fibers by coupling the dye to a dye site receptor which, in turn, is attached to the fiber. The process includes first swelling the fiber in a strong polar solvent and, while the fiber is in the swollen condition, introducing a substance capable of forming a strong chemical bond with an anionic dye into the swollen fiber. This dye site receptor substance is an amine, typically hexamethylenediamine. The procedure described requires at least three steps: first pretreating the fiber in a solution of solvent/swelling agent; treating with the diamine and a wetting agent; then drying to shrink the fiber and incorporate the diamine dye site receptor into the fiber. The thus-pretreated fabric is then dyed with an anionic dye. Aramid fibers described and purported to be successfully dyed in U.S. Pat. No. 4,198,494 are sold under the trademarks Nomex.RTM. and Kevlar.RTM. by DuPont, and under the trademark Conex.RTM. by Teijin Limited of Tokyo, Japan.
A process has been used by Cates and others in commonly-assigned U.S. patent application Ser. No. 870,523 filed June 4, 1986, now U.S. Pat. No. 4,710,200 for the continuous or semi-continuous dyeing of and simultaneous improving the flame-resistant properties of poly(m-phenyleneisophthalamide) fibers that includes the step of introducing the fiber into a fiber swelling agent solution also containing at least one dye together with at least one flame retardant, thereby swelling the fiber and introducing both the dye and the flame retardant into the fiber while in the swollen state. The flame resistance/performance properties of fabrics dyed by this process are significant. LOI values, as described below, may be as high as 44% for simultaneously dyed and flame retarded T-455 Nomex fabric product produced by the process of this invention. As a means of comparison, undyed T-455 Nomex has an LOI of 27%. However, this process involves some equipment not routinely available on most existing processing lines.
It is an object of the present invention to provide a process for dyeing an aramid fiber such as Nomex.RTM.. It is also an object to provide a process for simultaneously dyeing and not detracting from the inherent strength of the aramid fibers. It is also an object to provide a process suitable to conventional equipment such as pressure jets, or similar machines. It is particularly an object to provide a process for the preparation of dyed, "super FR" Nomex.RTM. fabrics of high LOI of 37%-44% as described in the Cates et al patent application.